1. Technical Field
This invention relates to a field of digital signal processing, and more specifically to a digital radio frequency RF) modulator, which is used for modulation of an RF sinusoidal carrier with a base band TV signal.
2. Description of Related Art
Modulation can be defined as an alteration of some characteristic of a known signal or waveform, i.e., a carrier, as a function of some unknown signal or waveform that conveys information. In radio-frequency (RF) communication systems, the carrier is typically a sinusoid, and there are several methods of modulating the carrier. These include linear modulation, angle modulation, and various types of pulse modulation. As defined in the ITU-R BT.470 standard, the standard definition (SD) video signal AM-VSB (Amplitude Modulation—Vestigial Side Band) modulates a vision carrier and the audio signal FM (frequency modulation) modulates an audio carrier.
In conventional systems the pulse code modulation (PCM) audio signal frequency modulates a carrier whose frequency is the difference in frequency between the vision carrier and the audio carrier. The frequency modulated carrier is added to a base band video signal. The resulting signal is then shifted to an intermediate frequency, called an IF frequency. At the IF frequency the signal is vestigial side band (VSB) filtered and then translated to a desired channel frequency for broadcasting.
In earlier systems, the entire transmission pipeline is implemented in an analog domain using analog signal processing concepts. With the advancement of digital VLSI design and evolution of System-On-Chip concepts, it was desired to reduce the system cost by converging the functionality of discrete on-board components into cost effective integrated circuits (ICs). As a first step in this direction, today the pipeline up to the IF stage has been widely replaced by digital components, followed by a D/A converter and an analog RF converter circuit to the desired TV channel frequency. To implement the RF converter block using digital logic, a very high sampling rate is required. Digital logic operated at a higher sampling rates have the problems of higher power consumption, difficulty in meeting timing requirements and a higher circuit area. Thus, it is difficult to replace the RF converter circuit by digital components.
Thus, there is a need for a digital RF modulator which operates in a digital domain for modulating the base band TV signals to a desired channel frequency without requiring an analog up conversion.